To establish a rapid and versatile method for estimating structural distributions of compositions in heavy oils, an aromatic ring size distribution is evaluated from a boiling point distribution obtained by a simulated distillation on the basis of our relationship between boiling points and chemical structures for heavy oils. To obtain the boiling point distribution, two simulated distillations for heavy oils were examined. One is a chromatographic method with a conventional GC or a supercritical fluid chromatography(SFC). The other is based on our boiling point calculation, which is related with chemical structures of heavy oils. Similarity of both simulated distillation curves suggests that the simulated distillation of the heavy oil by SFC could be available for a characterization of the heavy oil including nonvolatile materials. An evaluation method for a ring size distribution is proposed based on the boiling point distribution obtained from the simulated distillations. There was a difference between an actual boiling point distribution of the heavy oil and an imaginary one calculated on the assumption that all of the components were straight alkanes. The temperature difference (Delta T) between both distributions at the same distillate content is attributed to non-paraffinic structures in the heavy oil. As compared the aromatic ring size distribution evaluated by the temperature difference with the distribution analyzed by HPLC-MS measurement, both aromatic size distributions were in good agreement each other. (C) 2000 Elsevier Science Ltd. All rights reserved.